Propellants including two base components, such as a nitrocellulose (NC) and an energetic plasticizer, are commonly referred to as so-called “double base” propellants and are widely used in munitions, such as rifle and pistol cartridges, rocket motors, mortar shells, shotgun shells and missiles. Examples of energetic plasticizers that may be combined with the nitrocellulose to form the double base propellant include, but are not limited to, nitroglycerine, butanetriol trinitrate and diglycol dinitrate. The nitrocellulose desensitizes the highly unstable energetic plasticizer, preventing the double base propellant from detonating as a high explosive. The energetic plasticizer gelatinizes the nitrocellulose, increasing the energy density of the double base propellant. For example, conventional double base propellants may include, as main ingredients, between about 10% by weight (wt %) and about 90 wt % nitrocellulose and between about 10 wt % and about 90 wt % nitroglycerine. Such double base propellants may be loaded within a cartridge or shell casing used in an ordnance element, along with a primer composition used to initiate or ignite the double base propellant. The double base propellants may also be used in rocket motors and missiles, where they are disposed inside a case to provide thrust upon burning.
For ballistic applications, it is desirable for propellants to burn at a controlled and predictable rate without performance loss. Controlling the ballistic properties of the propellant, such as burn rate, enables proper function of the ordnance element or rocket motor. When the burn rate of the propellant is too high, pressures within the cartridge, shell casing or rocket motor case may exceed design capability, resulting in damage to or destruction of the cartridge, shell casing or rocket motor case. On the other hand, if the burn rate of the propellant is too low, the propellant may not provide sufficient velocity to propel a projectile of the ordnance element or the rocket motor over a desired course.
To tailor the ballistic properties of the propellant, such as the burn rate and the velocity, materials that control ballistic properties, so-called “ballistic modifiers,” may be included in the propellant. Various organometallic salts and various oxides have been used to modify the ballistic properties of propellants, such as double base propellants. Examples of such ballistic modifiers include lead-based compounds, such as, lead salts and lead oxides (e.g., lead salicylate, lead β-resorcylate and lead stearate). The use of lead-based compounds as ballistic modifiers poses a concern for the environment and for personal safety due to the toxic nature of lead when introduced into the atmosphere by propellant manufacture, rocket motor firing and disposal. The presence of these lead-based ballistic modifiers is, therefore, detrimental to the environment when the propellant is burning.
Conventional propellants may also contain ammonium perchlorate (AP), which upon combustion produces the toxic substance hydrochloric acid (HCl). Chloride ions released from hydrochloric acid in the upper atmosphere may react with and destroy ozone.
Other, nontoxic compounds have been investigated as potential replacements for lead-based ballistic modifiers in propellants. For example, copper- and barium-based compounds have been shown to modify the ballistic properties of propellants. However, performance characteristics of the propellants are impaired by the use of these copper- and barium-based compounds. Solid propellants containing copper salts as the ballistic modifier may exhibit a poor aging. Barium salts, being highly soluble in water, are problematic in conventional manufacturing processes used to form the propellants.
Red phosphorus has been investigated as a component in primer compositions for military applications. Red phosphorus is an allotrope of phosphorus that has a network of tetrahedrally arranged groups of four phosphorus atoms linked into chains. White phosphorous is another allotrope that is much more reactive and toxic than red phosphorus. The two allotropes have such unique physical characteristics that they have different CAS numbers, as registered by the Chemical Abstract Service (“CAS”). Red phosphorus is relatively stable in air and is easier to handle than other allotropes of phosphorus. However, if red phosphorus is exposed to oxygen (O2), water (H2O), or mixtures thereof at elevated temperatures, such as during storage, the red phosphorus reacts with the oxygen and water, releasing phosphine (PH3) gas and phosphoric acids (H3PO2, H3PO3, or H3PO4). As is well known, the phosphine is toxic and the phosphoric acids are corrosive. To improve the stability of red phosphorus in environments rich in oxygen or water, dust suppressing agents, stabilizers, or microencapsulating resins have been used. The dust suppressing agents are liquid organic compounds. The stabilizers are typically inorganic salts, such as metal oxides. The microencapsulating resins are thermoset resins, such as epoxy resins or phenolic resins. Currently, microencapsulating resins are not used in military applications. The military specification for phosphorous has been deactivated and is not expected to be updated to include encapsulation.
U.S. Pat. No. 7,857,921 to Busky et al. discloses a primer composition that includes a stabilized, encapsulated red phosphorus and combinations of at least one oxidizer, at least one secondary explosive composition, at least one light metal, or at least one acid resistant binder. The stabilized, encapsulated red phosphorus may include particles of red phosphorus, a metal oxide coating, and a polymer layer.